Since gallium nitride (GaN)-based light-emitting diodes (LED) have high luminous efficiency, they have been widely applied in backlight, lightning, landscaping, etc. as a light source. Currently, the focus of LED industry is to further improve the luminous efficiency of LED chips. Generally, the luminous efficiency can be determined by the following factors: (1) electron-hole radiative recombination efficiency in an active region (also known as internal quantum efficiency); and (2) light extraction efficiency.
A blue LED material, such as gallium nitride (GaN)-based material, is conventionally formed on a sapphire substrate. However, the sapphire substrate and the GaN-based material have a great difference in the lattice, which may result in significant dislocation (i.e., crystallographic defect or irregularity) within the resultant epitaxial crystal structure, and the formation of non-radiative recombination centers, thereby reducing the internal quantum efficiency of the LED. In addition, since the GaN-based materials have a high refractive index, light emitted from the active region of LED would be totally internally reflected, thereby decreasing the light extraction efficiency.
A patterned sapphire substrate may ameliorate the abovementioned deficiencies not only by effectively controlling the density of nucleation islands to improve the quality of the gallium nitride (GaN) crystal, but also by effectively scattering light emitted from the active region to suppress total internal reflection.
The patterned sapphire substrate is usually obtained by dry etching or wet etching the sapphire substrate. Referring to FIG. 1, in the dry etching, a substrate 10 is etched using physical bombardment to form a plurality of protrusions 11 (i.e., pattern), and the side walls 12 of the protrusions 11 may be damaged to form surface defects 13 (i.e., rough surface), which may lead to formation of crystal defects 32 in a subsequent step of epitaxial growth of an epitaxial staked structure 30 (as shown in FIGS. 2 and 3). Thus, the density of the defective epitaxial material is increased, which reduces internal quantum efficiency. Furthermore, the crystal defects 32 absorb light, thereby reducing light extraction efficiency.
Chinese Invention Patent Application Publication No. CN 104246980 A discloses that formation of an aluminum nitride (AlN) buffer layer between a substrate and an undoped and/or doped gallium nitride layer by physical vapor deposition (PVD) can improve the quality of the GaN crystal and the luminous efficiency of the resultant GaN-based light device.
However, there is still a need to provide a semiconductor device with high-quality epitaxial growth (i.e., less crystal defects) on a patterned substrate.